I have two UPS's (inverter with charger) that uses IC SG3525 to generate the PWM signal... The inverter's input is 12V battery and output is 220VAC.

The problem is that the output voltage is not controlled by feedback, meaning that when the battery voltage is say 12.8V the output is 285V as tested, and as the voltage drops the output voltage decreases. Also, as the load increase the output voltage decrease.

I know the buyer very well, he told me he have about 200pcs, so if I found a solution for this problem I could help him very much.

The PCB should be studied, but before, as I know there should be a feedback winding in the transformer, which I can't find in the inverter. The picture clarify it better.

The circuit uses the opto-coupler 4N35 as clear in the picture, it's fed using a 1KΩ resistor after a 4-diode bridge, so I don't think the voltage on the bridge output is high voltage... Reverse engineering the PCB is hard and will take long time, so if the pictures are not enough, ask me about the components connections and I will help you.

Note that I flip rotated one of the two pictures so they became on the same directions, easier to figure out.